1. when the ps4/ps3/pc is plugged into the adapter in a different room via ethernet wifi signal on every other device drops to around 2-3 mbps while the plugged in device stays at full speed. is this normal? to get the wfi speed to go back up to normal i need to unplug the cable from what ever device it is attached to.
If the device connected via ethernet and is downloading, then it will take the bandwidth away from the devices over Wireless yes.
IE you have 40Mbps speed, if the ethernet device is downloading at 40Mbps, it is using all the available speed... So yes I would expect a slowdown.
2 - 3Mbps is quite substantial though - but it depends what is being downloaded and how it is being downloaded.
2. like i said while the ps4 is plugged into the adapter in a different room the speed is fine and things download in minutes but if i am playing a shooter game online on the ps4 when its plugged in there seems to be some kind of lag and my bullets dont hit its target ( there is no visible lag or stutter in the game but its just like no bullets are hitting the target) even though the download and upload are fine and so is my signal to the game. where as this doesnt happen on the slower wifi speed.
The homeplugs may have a higher latency. Speed and latency are two different things. Virgin media can often be 150Mbps but perform worse in an online game than somebody with a 3Mbps connection due to latency. Homeplugs are probably not the best device to use for gaming.